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1.  Pre-Clinical studies of Notch Signaling Inhibitor RO4929097 in Inflammatory Breast Cancer Cells 
Basal breast cancer, common among patients presenting with inflammatory breast cancer, has been shown to be resistant to radiation and enriched in cancer stem cells. The Notch pathway plays an important role in self-renewal of breast cancer stem cells and contributes to inflammatory signaling that promotes the breast cancer stem cell phenotype. Herein we inhibited Notch signaling using a gamma secretase inhibitor, RO4929097, in an in vitro model that enriches for cancer initiating cells (3D clonogenic assay) and conventional 2D clonogenic assay to compare the effect on radiosensitization of the SUM149 and SUM190 inflammatory breast cancer (IBC) cell lines. RO4929097 downregulated the Notch target genes Hes1, Hey1 and HeyL and showed a significant reduction in anchorage independent growth in SUM190 and SUM149. However, the putative self-renewal assay mammosphere formation efficiency was increased with the drug. To assess radiosensitization of putative cancer stem cells, cells were exposed to increasing doses of radiation with or without 1uM RO4929097 in their standard (2D) and self-renewal enriching (3D) culture conditions. In the conventional 2D clonogenic assay, RO4929097 significantly sensitized SUM190 cells to ionizing radiation and has a modest radiosensitization effect in SUM149 cells. In the 3D clonogenic assays, however, a radioprotective effect was seen in both SUM149 and SUM190 cells at higher doses. Both cell lines express IL-6 and IL-8, cytokines known to mediate the efficacy of notch inhibition and to promote self-renewal of stem cells. We further showed that RO429097 inhibits normal T-cell synthesis of some inflammatory cytokines, including TNF-α, a potential mediator of IL-6 and IL-8 production in the microenvironment. These data suggest additional targeting agents may be required to selectively target IBC stem cells through notch inhibition, and that evaluation of microenvironmental influences may shed further light on the potential effects of this inhibitor.
PMCID: PMC4545650  PMID: 22547109
Notch; Cancer stem cells; R04929097; Inflammatory breast cancer; Radiation
2.  Inflammation Mediated Metastasis: Immune Induced Epithelial-To-Mesenchymal Transition in Inflammatory Breast Cancer Cells 
PLoS ONE  2015;10(7):e0132710.
Inflammatory breast cancer (IBC) is the most insidious form of locally advanced breast cancer; about a third of patients have distant metastasis at initial staging. Emerging evidence suggests that host factors in the tumor microenvironment may interact with underlying IBC cells to make them aggressive. It is unknown whether immune cells associated to the IBC microenvironment play a role in this scenario to transiently promote epithelial to mesenchymal transition (EMT) in these cells. We hypothesized that soluble factors secreted by activated immune cells can induce an EMT in IBC and thus promote metastasis. In a pilot study of 16 breast cancer patients, TNF-α production by peripheral blood T cells was correlated with the detection of circulating tumor cells expressing EMT markers. In a variety of IBC model cell lines, soluble factors from activated T cells induced expression of EMT-related genes, including FN1, VIM, TGM2, ZEB1. Interestingly, although IBC cells exhibited increased invasion and migration following exposure to immune factors, the expression of E-cadherin (CDH1), a cell adhesion molecule, increased uniquely in IBC cell lines but not in non-IBC cell lines. A combination of TNF-α, IL-6, and TGF-β was able to recapitulate EMT induction in IBC, and conditioned media preloaded with neutralizing antibodies against these factors exhibited decreased EMT. These data suggest that release of cytokines by activated immune cells may contribute to the aggressiveness of IBC and highlight these factors as potential target mediators of immune-IBC interaction.
PMCID: PMC4514595  PMID: 26207636
3.  Treatment With Lenalidomide Modulates T-Cell Immunophenotype and Cytokine Production in Patients With Chronic Lymphocytic Leukemia 
Cancer  2011;117(17):3999-4008.
Lenalidomide, an immunomodulatory agent, has activity in lymphoproliferative disorders. The authors, therefore, evaluated its effects on T-cell immunophenotype and cytokine production in patients with chronic lymphocytic leukemia (CLL).
To study the immunomodulatory effects of lenalidomide in CLL, the authors recruited 24 patients with untreated CLL enrolled in a phase 2 clinical trial of lenalidomide and obtained peripheral blood specimens for immunologic studies consisting of enumeration of T cells and assessing their ability to synthesize cytokines after activation through T-cell receptor (TCR).
After 3 cycles of therapy, patients had a significant reduction in percentage (%) and absolute lymphocyte count (ALC) and an increase in percentage of T cells, percentage of activated CD8+ T cells producing IFN-γ, and percentage of regulatory T (TR) cells when compared with their respective levels before treatment. After 15 cycles of treatment, responder patients had significant reduction in percentage of lymphocytes and ALC, percentage of activated CD4+ T cells producing IL-2, IFN-γ, or TNF-α, and percentage of TR cells when compared with their perspective levels after 3 cycles of treatment. Furthermore, the numbers of activated CD4+ T cells producing IL-2, IFN-γ, or TNF-α, activated CD8+ T cells producing IFN-γ, and TR cells normalized to the range of healthy subjects.
Treatment with lenalidomide resulted in the normalization of functional T-cell subsets in responders, suggesting that lenalidomide may modulate cell-mediated immunity in patients with CLL.
PMCID: PMC4349201  PMID: 21858802
chronic lymphocytic leukemia; immunomodulatory agents; T cells; cytokines
4.  Inflammatory markers and development of symptom burden in patients with multiple myeloma during autologous stem cell transplantation 
Increasing research suggests that inflammation mediates symptom development. In this longitudinal study, we examined inflammatory factors related to the development of high symptom burden during autologous stem cell transplant (AuSCT) for multiple myeloma.
Experimental Design
Patients (n = 63) repeatedly reported symptom severity on the M. D. Anderson Symptom Inventory multiple myeloma module (MDASI-MM) and contributed blood samples periodically for up to 100 days post-AuSCT for inflammatory marker assays. The temporal associations between serum inflammatory marker concentrations and symptom severity outcomes were examined by nonlinear mixed-effect modeling.
Fatigue, pain, disturbed sleep, lack of appetite, and drowsiness were consistently the most-severe MDASI-MM symptoms during the study. Peak symptom severity occurred on day 8 post-AuSCT, during white blood cell count nadir. Patterns of serum IL-6 (peak on day 9) and sIL-6R (nadir on day 8) expression paralleled symptom development over time (both P < 0.0001). By univariate analysis, serum IL-6, sIL-6R, IL-10, CRP, MIP-1α, sIL-1R2, sIL-1RA, and sTNF-R1 were significantly related to the most-severe symptoms during the first 30 days post-AuSCT (all P < 0.05). By multivariate analysis, IL-6 (estimate = 0.170, P = 0.004) and MIP-1α (estimate = −0.172, P = 0.006) were temporally associated with the severity of the component symptom score.
Systemic inflammatory response was associated with high symptom burden during the acute phase of AuSCT. Additional research is needed to understand how the inflammatory response is mechanistically associated with symptom expression and whether suppression of this response can reduce symptoms without compromising tumor control.
PMCID: PMC3947467  PMID: 24423611
symptom; inflammation; AuSCT (autologous stem cell transplant); MDASI; multiple myeloma
5.  Circulating tumor cells as early predictors of metastatic spread in breast cancer patients with limited metastatic dissemination 
Traditional factors currently used for prognostic stratification do not always adequately predict treatment response and disease evolution in advanced breast cancer patients. Therefore, the use of blood-based markers, such as circulating tumor cells (CTCs), represents a promising complementary strategy for disease monitoring. In this retrospective study, we explored the role of CTC counts as predictors of disease evolution in breast cancer patients with limited metastatic dissemination.
A total of 492 advanced breast cancer patients who had a CTC count assessed by CellSearch prior to starting a new line of systemic therapy were eligible for this analysis. Using the threshold of 5 CTCs/7.5 ml of blood, pretreatment CTC counts were correlated in the overall population with metastatic site distribution, evaluated at baseline and at the time of treatment failure, using Fisher’s exact test. Time to visceral progression and time to the development of new metastatic lesions and sites were estimated in patients with nonvisceral metastases and with single-site metastatic disease, respectively, by the Kaplan-Meier method. Survival times were compared between groups according to pretreatment CTC count by logrank test.
In the overall population, a pretreatment level ≥5 CTCs/7.5 ml was associated with an increased baseline number of metastatic sites compared with <5 CTCs/7.5 ml (P = 0.0077). At the time of treatment failure, patients with ≥5 CTCs/7.5 ml more frequently developed new metastatic lesions and sites compared with those with <5 CTCs/7.5 ml (development of new lesions: P = 0.0002; development of new sites: P = 0.0031). Among patients with disease originally confined to nonvisceral sites, ≥5 CTCs/7.5 ml was associated with remarkably shorter time to visceral metastases (P = 0.0021) and overall survival (P = 0.0006) compared with <5 CTCs/7.5 ml. In patients with single-site metastatic disease, ≥5 CTCs/7.5 ml was associated with a significant reduction of the time to development of new metastatic sites (P = 0.0051) and new lesions (P = 0.0002) and with worse overall survival (P = 0.0101).
Our results suggest that baseline CTC counts can be used as an early predictor of metastatic potential in breast cancer patients with limited metastatic dissemination.
Electronic supplementary material
The online version of this article (doi:10.1186/s13058-014-0440-8) contains supplementary material, which is available to authorized users.
PMCID: PMC4303121  PMID: 25223629
6.  High Serum miR-19a Levels Are Associated with Inflammatory Breast Cancer and Are Predictive of Favorable Clinical Outcome in Patients with Metastatic HER2+ Inflammatory Breast Cancer 
PLoS ONE  2014;9(1):e83113.
Altered serum microRNA (miRNA) levels may be correlated with a dysregulated expression pattern in parental tumor tissue and reflect the clinical evolution of disease. The overexpression of miR-21, miR-10b, and miR-19a is associated with the acquisition of malignant characteristics (increased tumor cell proliferation, migration, invasion, dissemination, and metastasis); thus, we determined their utility as serum biomarkers for aggressive breast cancer (HER2-overexpressed or -amplified [HER2+] and inflammatory breast cancer [IBC]).
Experimental Design
In this prospective study, we measured miR-21, miR-10b, and miR-19a levels using quantitative reverse transcriptase-polymerase chain reaction in the serum of 113 breast cancer patients and determined their association with clinicopathologic factors and clinical outcome. Thirty healthy donors with no history of cancer were enrolled as controls.
Patients with non-metastatic HER2+ breast cancer had higher serum miR-21 median levels than patients with non-metastatic HER2− disease (p = 0.044); whereas patients with metastatic HER2+ breast cancer had higher serum miR-10b median levels than patients with metastatic HER2− disease (p = 0.0004). There were no significant differences in serum miR-19a median levels between HER2+ and HER2− groups, regardless of the presence of metastases. High serum miR-19a levels were associated with IBC (p = 0.039). Patients with metastatic IBC had significantly higher serum miR-19a median levels than patients with metastatic non-IBC (p = 0.019). Finally, high serum miR-19a levels were associated with longer progression-free survival time (10.3 vs. 3.2 months; p = 0.022) and longer overall survival time (median not reached vs. 11.2 months; p = 0.003) in patients with metastatic HER2+ IBC.
High levels of miR-21 and miR-10b were present in the serum of patients with non-metastatic and metastatic HER2+ breast cancer, respectively. High levels of serum miR-19a may represent a biomarker for IBC that is predictive for favorable clinical outcome in patients with metastatic HER2+ IBC.
PMCID: PMC3885405  PMID: 24416156
7.  The Role of Thalidomide and Placebo for the Treatment of Cancer-Related Anorexia-Cachexia Symptoms: Results of a Double-Blind Placebo-Controlled Randomized Study 
Journal of Palliative Medicine  2012;15(10):1059-1064.
To determine the effects of thalidomide and placebo on anorexia-cachexia and its related symptoms, body composition, resting metabolic rate, and serum cytokines and their receptors in patients with advanced cancer.
Included in the study were patients with advanced cancer with weight loss greater than 5% in 6 months and who reported anorexia, fatigue, and one of the following: anxiety, depression, or sleep disturbances. Patients on chemotherapy within 2 weeks prior or during the study were excluded from the study. Patients were randomly assigned to either 100 mg thalidomide or placebo once a day for 14 days. The Edmonton Symptom Assessment Scale (ESAS), Functional Assessment of Anorexia/Cachexia Therapy (FAACT), Functional Assessment of Cancer Illness Therapy (FACIT-F), Hospital Anxiety Depression Scale (HADS) Pittsburgh Sleep Quality Index (PSQI) were utilized, and in addition body composition, Resting Energy Expenditure (REE), and serum cytokine levels were assessed.
Of the 31 patients entered in the study, 15 were assigned to the thalidomide group and 16 to the placebo group. However only 21/31 patients were able to complete the study. Compared with their baseline values, both the thalidomide and the placebo groups showed significant reduction in cytokines. Tumor necrosis factor (TNF)-α (p=0.04) and its receptors TNFR1 (p=0.04), TNFR2 (p=0.04), and interleukin (IL)-8 (p=0.04) were statistically significant in the thalidomide group. In the placebo group, TNF-α (p=0.008), TNFR1 (p=0.005), TNFR2 (p=0.005), IL-RA (p=0.005), IL-6 (p=0.005), and IL-8 (p=0.005) were statistically significant. However, improvement in these symptoms and cytokine levels were not significantly different in the thalidomide group compared with the placebo group. None of the patients withdrew from the study because of toxicity of either thalidomide or placebo.
Based on the poor accrual rate and attrition observed in this study, it is important that future research on thalidomide as a treatment for cancer-related anorexia-cachexia symptoms (ACS) in patients with advanced cancer use less stringent entry criteria and less exhaustive outcome measures.
PMCID: PMC3438834  PMID: 22880820
8.  Associations of Cytokines, Sleep Patterns, and Neurocognitive Function in Youth with HIV Infection 
Youth infected with HIV at birth often have sleep disturbances, neurocognitive deficits, and abnormal psychosocial function which are associated with and possibly resulted from elevated blood cytokine levels that may lead to a decreased quality of life. To identify molecular pathways that might be associated with these disorders, we evaluated 38 HIV-infected and 35 uninfected subjects over 18-months for intracellular cytokine levels, sleep patterns and duration of sleep, and neurodevelopmental abilities. HIV infection was significantly associated with alterations of intracellular pro-inflammatory cytokines (TNF-α, IFN-γ, IL-12), sleep factors (total time asleep and daytime sleep patterns), and neurocognitive factors (parent and patient reported problems with socio-emotional, behavioral, and executive functions; working memory-mental fatigue; verbal memory; and sustained concentration and vigilance. By better defining the relationships between HIV infection, sleep disturbances, and poor psychosocial behavior and neurocognition, it may be possible to provide targeted pharmacologic and procedural interventions to improve these debilitating conditions.
PMCID: PMC3377781  PMID: 22659030
pediatric HIV infection; intracellular cytokines; sleep behavior; neurodevelopment; neurocognition; path analysis
9.  Expression of Epithelial-Mesenchymal Transition-Inducing Transcription Factors in Primary Breast Cancer: The Effect of Neoadjuvant Therapy 
Epithelial cancer cells are likely to undergo epithelial mesenchymal transition (EMT) prior to entering the peripheral circulation. By undergoing EMT, circulating tumor cells (CTCs) lose epithelial markers and may escape detection by conventional methods. Therefore, we conducted a pilot study to investigate mRNA transcripts of EMT-inducing transcription factors (TFs) in tumor cells from the peripheral blood (PB) of primary breast cancer (PBC) patients.
Peripheral blood mononuclear cells were isolated from 52 stages I–III PBC patients and 30 healthy donors (HD) and sequentially depleted of EpCAM+ cells and CD45+ leukocytes, henceforth referred to as CD45−. The expression levels of EMT-inducing TFs (TWIST1, SNAIL1, SLUG, ZEB1, and FOXC2) in the CD45− cells were determined using qRT-PCR. The highest level of expression by the CD45− cell fraction of HD was used as “cut off” to determine if samples from PBC patients overexpressed any EMT-inducing TFs. In total, 15.4% of PBC patients overexpressed at least one of the EMT-inducing TF transcripts. Overexpression of any EMT-inducing TF transcripts was more likely to be detected in PBC patients who received neoadjuvant therapies (NAT) than patients who received no NAT (P = 0.003). Concurrently, CTCs were detected in 7 out of 38 (18.4%) patients by CellSearch® and 15 out of 42 (35.7%) patients by AdnaTest™. There was no association between the presence of CTCs measured by CellSearch® or AdnaTest™.
In summary, our results demonstrate that CTCs with EMT phenotype may occur in the peripheral circulation of PBC patients and NAT is unable to eliminate CTCs undergoing EMT.
PMCID: PMC3169728  PMID: 21387303
circulating tumor cells; epithelial-mesenchymal transition; primary breast cancer; neoadjuvant therapy
10.  Primary breast cancer patients with high risk clinicopathologic features have high percentages of bone marrow epithelial cells with ALDH activity and CD44+CD24lo cancer stem cell phenotype 
Cancer stem cells (CSCs) are purported to be epithelial tumor cells expressing CD44+CD24lo that exhibit aldehyde dehydrogenase activity (Aldefluor+). We hypothesized that if CSCs are responsible for tumor dissemination, disseminated cells in the bone marrow (BM) would be positive for putative breast CSC markers. Therefore, we assessed the presence of Aldefluor+ epithelial (CD326+CD45dim) cells for the presence of the CD44+CD24lo phenotype in BM of patients with primary breast cancer (PBC).
BM aspirates were collected at the time of surgery from 66 patients with PBC. Thirty patients received neoadjuvant chemotherapy (NACT) prior to aspiration. BM was analyzed for Aldefluor+ epithelial cells with or without CD44+CD24lo expression by flow cytometry. BM aspirates from 3 healthy donors (HD) were subjected to identical processing and analyses and served as controls.
Patients with triple-receptor-negative (TN) tumors had a significantly higher median percentage of CD44+CD24lo CSC within Aldefluor+ epithelial cell population than patients with other immunohistochemical subtypes (P=0.018). Patients with TN tumors or with pN2 or higher pathologic nodal status were more likely to have a proportion of CD44+CD24lo CSC within Aldefluor+ epithelial cell population above the highest level of HD. Furthermore, patients who received NACT were more likely to have percentages of Aldefluor+ epithelial cells greater than the highest level of HD (P=0.004).
The percentage of CD44+CD24lo CSC in the BM is higher in PBC patients with high risk tumor features. The selection or enrichment of Aldefluor+ epithelial cells by NACT may represent an opportunity to target these cells with novel therapies.
PMCID: PMC3116032  PMID: 21334874
11.  Mesenchymal stem cells expressing GD2 and CD271 correlate with breast cancer-initiating cells in bone marrow 
Cancer Biology & Therapy  2011;11(9):812-815.
The bone marrow microenvironment is considered a critical component in the dissemination and fate of cancer cells in the metastatic process. We explored the possible correlation between bone marrow mesenchymal stem cells (BM-MSC) and disseminated breast cancer-initiating cells (BCIC) in primary breast cancer patients.
The percentages of BCIC (Aldefluor+CD326+CD44+CD24−) correlated with the percentages of BM-MSC, either CD45−GD2+CD200+CD271+ (Kedall's τ = 0.684, p = 0.004) or CD45−GD2+CD271+ in the bone marrow (Kedall's τ = 0.464, p = 0.042).
Experimental Design
Bone marrow mononuclear cells (BM-MNC) were collected at the time of primary surgery in 12 breast cancer patients. BM-MNC was immunophenotyped and BCIC was defined as epithelial cells (CD326+CD45−) with a “stem-like” phenotype (CD44+CD24low/−, ALDH activity). BM-MSC was defined as CD34−CD45− cells that co-expressed GD2, CD271 and/or CD200 within CD326-depleted BM-MNC.
There was a positive correlation between mesenchymal stem cells expressing GD2 and CD271 and breast cancer-initiating cells in BM of patients with primary breast cancer.
PMCID: PMC3230296  PMID: 21358274
mesenchymal stem cells; bone marrow; microenvironment; cancer-initiating cells; cancer stem cells
12.  Prognostic Value of EMT-Circulating Tumor Cells in Metastatic Breast Cancer Patients Undergoing High-Dose Chemotherapy with Autologous Hematopoietic Stem Cell Transplantation 
Journal of Cancer  2012;3:369-380.
Background: Circulating tumor cells (CTCs) are an independent prognostic factor in metastatic breast cancer (MBC) patients treated by conventional dose chemotherapy. The aim of this study was to determine the role of CTCs and CTCs undergoing epithelial-mesenchymal transition (EMT) in metastatic breast cancer. We used the platform of high-dose chemotherapy (HDCT) and autologous hematopoietic stem cell transplantation (AHSCT) to study the CTCs and CTCs with EMT.
Patients and methods: CTCs were enumerated in 21 MBC patients before apheresis and 1 month after AHSCT. CD34-depleted apheresis products were analyzed for CD326+ epithelial and Aldefluor+ cancer stem cells (CSC) by flow cytometry and were depleted of CD45+ cells and assessed for EMT-inducing transcription factors (EMT-TF) by quantitative RT-PCR.
Results: Patients with ≥ 5 CTCs/7.5 mL of peripheral blood 1 month after AHSCT had shorter progression-free survival (PFS) (P=0.02) and overall survival (OS) (P=0.02). Patients with apheresis products containing high percentages of CD326+ epithelial cells or overexpressing EMT-TF had shorter PFS. In multivariate analysis, low percentage of CD326+ epithelial cells and response to HDCT with AHSCT were associated with longer PFS, whereas lower CTCs after AHSCT was associated with longer OS. High CTCs, 1 month after AHSCT correlated with shorter PFS and OS in MBC patients undergoing HDCT and AHSCT, while CTCs with EMT and CSCs phenotype in apheresis products are associated with relapse.
Conclusion: Our data suggest that CTC and CTCs with EMT are prognostic in MBC patients undergoing HDCT followed by AHSCT.
PMCID: PMC3471078  PMID: 23074378
metastatic breast cancer; circulating tumor cells; epithelial-mesenchymal transition; high-dose chemotherapy; autologous hematopoietic stem cell transplantation.
13.  Phase I Dose Escalation Study of Sodium Stibogluconate (SSG), a Protein Tyrosine Phosphatase Inhibitor, Combined with Interferon Alpha for Patients with Solid Tumors 
Journal of Cancer  2011;2:81-89.
Purpose: Sodium stibogluconate (SSG), a small molecule inhibitor of protein tyrosine phosphatases, combined with IFN-alpha-2b (IFN-α) inhibited solid tumor cell line growth in vitro. We conducted a phase I clinical trial with SSG plus IFN-α in advanced cancer patients to assess tolerance, maximum tolerated dose (MTD) and immune system effects.
Experimental Design: SSG was administered intravenously alone for five days of week 1, cycle 1 (21 days per cycle) and together with IFN-α 2b s (3 million units sc TIW) in week 2, and after a rest during week 3, on a 2-week on/1-week off cycle. SSG dose levels were 400, 600, 900, 1125, and 1350 mg/m2.
Results: Twenty-four patients were studied. Common toxicities included asymptomatic elevated serum lipase, thrombocytopenia, fatigue, fever, chills and anemia. The dose-limiting toxicities (DLT) were hypokalemia, thrombocytopenia, fatigue, pancreatitis and skin rash. The MTD was 900 mg/m2 SSG and IFN-α, 3 million units TIW. At this dose, patients had a significantly lower number of regulatory T cells (TR Cells) (p = 0.012), myeloid dendritic cells (mDC) (p = 0.028); higher percentage of natural killer (NK) cells that synthesized perforin (p = 0.046) and of plasmacytoid dendritic cells (pDC) that secreted IFN-α (p = 0.018) in response to activation through toll-like receptor (TLR) 7 and TLR 8 by CL097, the highly water-soluble derivative of the imidazoquinoline compound R848.
Conclusions: SSG in combination with IFN-α 2b was well tolerated and augmented cellular immune parameters.
PMCID: PMC3039225  PMID: 21326629
sodium stibogluconate; phase 1; interferon alpha; immunity; cancer
15.  Modification of a Hydrophobic Layer by a Point Mutation in Syntaxin 1A Regulates the Rate of Synaptic Vesicle Fusion  
PLoS Biology  2007;5(4):e72.
Both constitutive secretion and Ca2+-regulated exocytosis require the assembly of the soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) complexes. At present, little is known about how the SNARE complexes mediating these two distinct pathways differ in structure. Using the Drosophila neuromuscular synapse as a model, we show that a mutation modifying a hydrophobic layer in syntaxin 1A regulates the rate of vesicle fusion. Syntaxin 1A molecules share a highly conserved threonine in the C-terminal +7 layer near the transmembrane domain. Mutation of this threonine to isoleucine results in a structural change that more closely resembles those found in syntaxins ascribed to the constitutive secretory pathway. Flies carrying the I254 mutant protein have increased levels of SNARE complexes and dramatically enhanced rate of both constitutive and evoked vesicle fusion. In contrast, overexpression of the T254 wild-type protein in neurons reduces vesicle fusion only in the I254 mutant background. These results are consistent with molecular dynamics simulations of the SNARE core complex, suggesting that T254 serves as an internal brake to dampen SNARE zippering and impede vesicle fusion, whereas I254 favors fusion by enhancing intermolecular interaction within the SNARE core complex.
Author Summary
Most living cells constantly renew their membrane compositions and frequently communicate with neighboring cells by delivering cargo molecules from small vesicles. A key step in cargo delivery requires the fusion of the vesicle membrane with the target membrane mediated by SNARE proteins. In most cellular compartments, fusion occurs constitutively, requiring little participation of other molecules. In other cellular compartments, such as synapses in the nervous system, vesicle fusion is predominantly triggered by intracellular calcium ions. At present, constitutive and regulated fusion modes are not well understood.
In this study, we found that a mutant SNARE protein, syntaxin at the synapse, contained a building block commonly conserved for syntaxins functioning along constitutive secretory pathways. Further, our modeling predicted that the mutant syntaxin could form a tightly packed SNARE bundle closely resembling that found in the endosome, but differing from the relatively loosely packed bundle found at the wild-type synapse. Our experimental data support the hypothesis that the mutant syntaxin lowered the energy barrier for vesicle fusion by tightening the SNARE bundle. These findings reveal a novel, intrinsic structural feature of the SNARE complex that regulates vesicle fusion rate at different cellular compartments.
A syntaxin 1A threonine to isoleucine mutation is found to enhance SNARE complex formation and vesicle fusion. This structural change results in a syntaxin that resembles those in constitutive secretory pathways.
PMCID: PMC1808484  PMID: 17341138

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